Generally, zeolite is a crystalline aluminum silicate which has microspores having a molecular size of 0.2-2 nm, and is widely used in various applications such as a catalyst, an adsorbent, an ion exchanger, an absorbent, etc. owing to its pore characteristics and ion exchange functions. Particularly, when the zeolite is used as a catalyst or a catalyst support, the pore characteristics can have large influence on the catalyst performance. In zeolites having microspores, the transfer of molecules through pores via diffusion is very slow, thereby making the reaction slow, increasing side reactions or reducing the catalyst life. In order to solve such problems of the microporous zeolite, several attempts have been made, by making the zeolite crystal size smaller for increasing the exterior surface area, by reducing the transfer distance within the zeolite crystal, or by forming mesopores inside the zeolite crystal so as to allow the molecular transfer only through the mesopores while allowing the reaction only occurred in the micro spores.
When zeolite has mesopores simultaneously with microspores, the catalyst performance is improved as compared to the zeolite having only microspores, and particularly there are several advantages such as lengthened catalyst life and improved selectivity. When zeolites only have microspores, the distance for reactants or products to pass through the zeolite microspores is long, and it may result in side reactions or coke generation, leading to decrease in catalyst life and diffusion rate of molecules. In the meantime, in zeolites having mesopores, reactions occur mainly in the microspores and the transfer of materials (reactants or products) can be rapidly conducted through the mesopores adjacent to the microspores, thereby making the retention time of materials in microspores shorten, which results in excellent catalyst selectivity and increased catalyst life owing to the reduced coke generation. Zeolites only having microspores have a characteristic of participating in the reaction only on the surface of zeolite crystals, while zeolites having mesopores can participate in the catalyst reaction as the whole zeolite crystal and thus have high utility.
Various approaches have been made to prepare zeolites having mesopores. For example, provided is a method comprised of: synthesizing a material having regular mesopores and the inner wall of which consists of amorphous silica (SiO2) or silica-alumina (SiO2—Al2O3); impregnating the resulted material with a zeolite-templating compound, an organic amine; and subjecting the resulted product to hydrothermal synthesis so as to transform the amorphous inner wall into a zeolite crystal, thereby obtaining a zeolite material having mesopores (Chemistry of Materials 13(2001), pp. 683-687, U.S. Pat. No. 6,669,924). As another method, similar to the above-mentioned method of prior arts, a method comprising the steps of: impregnating an amorphous material with mesopores with zeolite seeds, instead of organic amine zeolite-templating compound of the first mentioned method; and subjecting the mesoporous inner wall to hydrothermal synthesis for zeolite crystallization so as to provide mesoporous zeolite, has been described (Angewandte Chemie International, 41 (2002), P1036-1040). Still as an alternative method, a method of preparing mesoporous zeolites has been described wherein an organic material as a template material is mixed into a source for zeolite synthesis such as Al, Si or the like and the mixture is subjected to hydrothermal synthesis (EP patent No. 1,882,676). As a fourth example, a method for preparing mesoporous zeolite using a surfactant having organosilan group has been described, wherein the silan group is bonded to the zeolite surface, and the long chain hydrocarbon attached to the silan group contributes to the mesopore formation (WO2007/043731, registered Korean patent No. 727,288). As a fifth example, a method for preparing mesoporous zeolites has been described, wherein a template for mesopore formation such as activated charcoal is mixed with a synthetic zeolite gel and then the mixture is subjected to a hydrothermal synthesis (U.S. Pat. No. 6,565,826). As a sixth known example of prior arts, a method for introducing mesopores into zeolite crystals by dealumination or desilication of the zeolite has been described (U.S. Pat. No. 5,069,890, EP patent No. 0,528,494, U.S. Pat. No. 6,017,508).
However, the conventional methods as described above have problems such as complex process, high production cost and the like.